Method and device for producing at least partially open-celled foam films from styrene polymers

ABSTRACT

A method for producing an at least partially open-celled foam foil on a basis of styrene polymers, nucleating agents, surfactants, physical foaming agents and, if required, further additives, such as foreign polymers and auxiliary materials, comprising a twin-screw extruder with identical parallel screws rotating in the same direction, wherein the twin-screw extruder has a length of at least 25 D (D=screw diameter), in particular 30 to 42 D, with a draw-in and plastification zone of a length of 10 to 15 D, and a mixing zone of a length of 3 to 6 D, and a following cooling zone.

[0001] The invention relates to a method for producing an at least partially open-celled foam foil from styrene polymers, as well as a device for producing the same using a twin-screw extruder.

[0002] At least partially open-celled foam foils are used wherever absorption capabilities, for example, are required, in particular for fluids, or foils permeable to water vapor and air are desired. A preferred area of application for such partially open-celled foam foils made of styrene polymers is in connection with packaging and containers. Foam foils are also processed in many ways into end products, for example shells, by subsequent thermal deformation.

[0003] A method for producing an extruded, open-celled foam material on the basis of styrene polymers is known from WO 96/00258, wherein the polymer is melted in a single-screw extruder. The open-celled foam foils produced are used in particular for employment as insulating elements for roofs.

[0004] Furthermore, a method for producing an open-celled polystyrene foil for packaging shells is known from EP 0 849 309 A1, wherein a mixture of polystyrene, which contains incorporated aliphatic hydrocarbons with 4 to 6 carbon atoms and, if required, additional polystyrene, as well as nucleating agents, is melted in a twin-screw extruder at temperatures between 130 to 150° C. and is extruded, wherein twin-screw extruders with an L/D ratio of 20 are employed.

[0005] The object of the invention is based on creating an efficient method for producing at least partially open-celled foam foils on the basis of styrene polymers, whose properties can be flexibly matched to various requirements, such as the degree of openess of the cells, density, thickness and polymer composition. It is in particular intended to create an open-celled foam structure which allows the passage of gases and fluids and allows a selectively adjustable, specific absorption of polar/non-polar fluids, as well as a specific absorption capability for polar fluids.

[0006] In accordance with the invention, this object is attained with a method in accordance with the characterizing features of claim 1.

[0007] The method in accordance with the invention for producing an at least partially open-celled foam foil on the basis of styrene polymers is distinguished in that a mixture, which contains 0.5 to 3 parts by weight of at least one nucleating agent, 0.5 to 3.5 parts per weight of at least one surfactant, and 1 to 3 parts per weight of a foreign polymer, which cannot be mixed with polystyrene, per 100 parts per weight of styrene polymer, is melted in a twin-screw extruder with screws rotating in the same direction at a temperature of up to 270° C. and 2 to 6 wt.-% of at least one physical foaming agent in relation to the amount of styrene polymer employed is injected into the polymer melt at an injection pressure, which lies above the cylinder pressure of the twin-screw extruder at the injection point and is higher than that for dissolving the foaming agent in the polymer, or sufficiently high for making possible a homogeneously distributed, two-phase system of melt/foaming agent, and the polymer melt is brought to a material temperature between 160 and 180° C. while being homogeneously mixed and prior to moving out of the twin-screw extruder into the nozzle tool with an annular nozzle, and is extruded under a pressure of up to 55 bar out of the nozzle into the atmosphere in the form of a tube, the tube is cooled on the inside and the outside while expanding and is pulled over a cooling and calibration mandrel, and is cut open after it has become sufficiently firm and is laid flat as the foamed foil.

[0008] The injection pressure lies in particular in the range between 30 and 160 bar.

[0009] Advantageous further developments of the invention can be taken from the characterizing features of the method claims.

[0010] A further object of the invention is a device for producing an at least partially open-celled foam foil on the basis of styrene polymers, nucleating agents, surfactants, physical foaming agents and, if required, further additives, such as foreign polymers and auxiliary materials, comprising a twin-screw extruder with a draw-in and plastification zone, mixing zone and cooling zone, wherein the styrene polymers with the nucleating agent, surfactant, additives are conducted, if required at least partially as a pre-mix, to the twin-screw extruder and are melted to form a polymer melt, and the physical foaming agent is injected under increased pressure into the polymer melt, the polymer melt already injected with gas is cooled to a temperature suitable for foaming and lying below the melt temperature of the styrene polymer, and thereafter exits into the atmosphere from the nozzle of the twin-screw extruder in the form of a tube while expanding, the tube is cooled and cut open in the longitudinal direction and laid flat as the foamed foil.

[0011] To attain the object in accordance with the invention, this device is further developed in accordance with the invention in that as the extruder for melting the polymer, mixing the polymer melt with the physical foaming agent and cooling the expandable polymer melt obtained in this manner, a twin-screw extruder of a length of at least 25 D, in particular 30 to 42 D and having identical parallel screws rotating in the same direction is provided, and the physical foaming agent can be introduced into the melted polymer by the conveying device at an injection pressure of less than 160 bar, in particular between 30 to 150 bar, at the end of the draw-in and plastification zone of an approximate length of 10 to 15 D of the twin-screw extruder and can be mixed with the molten polymer in the mixing zone of a length corresponding to 3 to 6 D of the twin-screw extruder adjoining the draw-in and plastification zone to form an expandable polymer melt.

[0012] Advantageous further developments of the device of the invention can be taken from the characterizing features of the dependent claims.

[0013] In accordance with the invention it is possible to create polystyrene foam with an open-celled foam structure for the passage of gases and fluids, and with a selectively adjustable specific absorption of polar and non-polar fluids. The absorption capability for fluids is a function of the surface tension of the retained fluid. With an initially high surfactant absorption, an initially high wetting effect results, however, the absorption capability for polar fluids rises with a decrease in the concentration of the surfactant at the cell wall. The surfactant is required for absorptive wetting of polar fluids.

[0014] The homo-, as well as the copolymers of styrene are employed as the styrene polymer for the foam foils to be produced, also mixtures or blends of polystyrene with other substances, as well as high-molecular substances produced by the polymerization of styrene by heat or with peroxide catalysts, such as products made by substance polymerization, as well as the copolymers of styrene with acrylonitrile and/or butadiene, or acrylonitrile and acrylic esters, or the graft polymers, wherein styrene is polymerized with the addition of natural or synthetic caoutchouc, such as styrene butadiene caoutchouc. Styrene homopolymers are preferred for the foam foils in accordance with the invention.

[0015] By means of the method in accordance with the invention it is achieved in a physical foaming process and with the use of twin-screw extruders to produce foam foils made of styrene polymers having a proportion of at least 25% of open cells. In the course of this, foam foils with a density of 40 to 100 kg/m³ or more are obtained, wherein foam foils of a thickness from 0.3 to 10 mm, preferably 1 to 5 mm, are produced. The foam foils preferably have a cell size in the range between 150 μm to 900 μm.

[0016] In accordance with the invention an extrusion process with an expansion process in the manner of a blow molding process is employed. The expandable polymer melt, which exits a ring-shaped nozzle of the extruder in the form of a tube, is immediately cooled on the inside and the outside, for example by means of cooling air, and is charged with pressure on the inside in order to achieve an expansion ratio greater than 1, in particular 1:3 to 1:5 and to calibrate the tube after expansion by means of a cooling and calibration mandrel and to cool it by contact with the cooling and calibration mandrel. It is possible in the course of this to subject the foam foil produced as a tube to a stretching process. The desired expansion ratio is primarily also a function of the desired thickness.

[0017] Organic foaming agents from the group of aliphatic hydrocarbons with 1 to 9 carbon atoms are particularly suitable as the physical foaming agents, such as methane, ethane, propane, n-butane, isobutane, N-pentane, isopentane and/or neopentane, and/or inorganic foaming agents such as carbon dioxide, nitrogen. It is possible here to either employ only a single foaming agent, or also a mixture of foaming agents. In the case of employing mixtures of foaming agents, i.e. when using two or more foaming agents, they are supplied to the polymer melt separately and directly one after the other. In this case the foaming agents are injected into the polymer melt by means of a pressure conveying device with a high degree of accuracy, metering device. It is essential for the invention that an injection pressure range is maintained, which preferably lies between 30 to 160 bar, between 30 to 70 bar is preferable. In this case the injection pressure is higher than the pressure above the cylinder pressure at the injection point, it furthermore needs to be higher than the pressure required for dissolving the foaming agent in the polymer, or sufficiently high for making possible a homogeneously distributed two-phase system of melt/foaming agent.

[0018] The amount of physical foaming agent is determined, on the one hand, by the desired density of the foam material and, on the other hand, also by the degree of cell openess. When using aliphatic hydrocarbons, in particular from the group n-butane, n- pentane, isopentane or mixtures thereof, 3 to 4 parts by weight of foaming agent per 100 parts by weight of styrene polymer have been shown to be suitable.

[0019] Polymer/foaming agent mixtures can be single-phase (foaming agent completely dissolved, or foaming agent partially dissolved, partially liquid) or multi-phase (melt liquid, foaming agent gaseous, finely dispersed, or foaming agent above the solution threshold is gaseous (CO₂/N₂)).

[0020] The open cell content can also be adjusted by means of the nucleating agent, wherein higher amounts of nucleating agent increase the proportion of open cells in the foam foil. Suitable nucleating agents are calcium carbonate, calcium stereate, talcum, titanium dioxide, silica, barium stereate, diatomaceous earth and/or mixtures of citric acid and sodium bicarbonate, wherein amounts of 0.6 to 1.5 parts by weight per 100 parts by weight of styrene polymer have been shown to be sufficient.

[0021] A high temperature profile, which causes high gas pressure at the same time, leads to openess of the cells when the cells have low wall strength.

[0022] A further option of encouraging openess of the cells and the proportion of open cells consists of the addition of a foreign polymer in small amounts, wherein foreign polymers which cannot be mixed with styrene polymer are used here, for example polyolefins or ethylene-vinyl acetate copolymer. Among the polyolefins, HDPE, LDPE and LLDPE, or also polyolefin waxes, are particularly suitable.

[0023] Interfaces with the foreign polymers form predetermined wetting points because of low surface tension.

[0024] If it is intended to increase the absorption capability and absorbency of the produced foam foil, it can be practical for increasing the absorption capability for aqueous fluids, in particular polar fluids, to add surfactants, in particular on the basis of alkane sulfonate, to the polymer mixture. It is also possible to employ surfactants with antistatic properties. The holding capacity of the fluids is a result of the surface tension of the absorbed fluid. Surfactants are required for absorbing polar fluids. The absorption capacity is defined and controlled primarily by the number of open cells, secondarily by the foam morphology and, in relation to the selective substance absorption, by the provision of a surfactant.

[0025] Further than that it is also possible to add to the polymer mixture auxiliary materials, such as inorganic fillers, pigments, antioxidants, UV absorbers, flame-protection agents, soot, anti-static agents, processing aids, lubricants and/or waxes in customary amounts.

[0026] Further than that it is also possible to employ the method of the invention with a polymer mixture with which up to 50 wt.-% or more of a regenerated styrene polymer have been used instead of fresh material.

[0027] An at least partially open-celled foam material foil, which is for example suitable for producing packaging shells, can be produced in accordance with the invention from a composition of 100 parts by weight of polystyrene, 3 to 4 parts by weight of foaming agent, selected from the group of n-butane, n-pentane, isopentane, or mixtures thereof, 0.6 to 1.5 parts by weight of nucleating agents from sodium bicarbonate and citric acid with a decomposition temperature of greater than 160° C.,

[0028] 1 to 3.5 parts by weight of a surfactant, which reacts well with polar substances such a polystyrene, and 1 to 3 parts by weight of a foreign polymer incapable of mixing with polystyrene, such as a polyolefin or ethylene-vinyl acetate copolymer. The foam foils produced in this way basically have a dense outer skin and an at least partially open-celled core layer, wherein this outer skin is smoother on the calibrated side than on the non-calibrated side. Absorption capability is provided in that one or both sides of the foam foil are needled or are opened by roughening or perforating. With thick foam foils it is also possible to split these, so that a foil is obtained which remains fluid-proof on one side, and on the other side is completely open for absorbing fluids.

[0029] Nucleating agents on the basis of sodium bicarbonate and citric acid, which are bound by means of a polymer binding agent on the basis of an acrylate polymer, preferably at a ratio of nucleating agent to polymer binding agent of 55:45 to 65:35, have been shown to be particularly advantageous for processing in a polymer mixture.

[0030] The production in accordance with the invention of an at least partially open-celled foam foil based on styrene polymers rests on the selection of a suitable mixture of raw materials, together with a specially designed twin-screw extruder with identical parallel screws rotating in the same direction, gas injection of a physical foaming agent at relatively low pressures. In particular, melting of the styrene polymer, injection of the physical foaming agent into the polymer melt, and cooling of the expandable polymer melt already injected with gas, which is obtained in this way, to the outlet temperature is performed in a twin-screw extruder with a draw-in and plastification zone, mixing zone and cooling zone and a ring-shaped outlet nozzle of a length of at least 25 D, in particular 30 to 42 D (D=screw diameter) with identical parallel screws rotating in the same direction, and the physical foaming agent is introduced into the polymer melt at an injection pressure, preferably lower than 160 bar, in particular between 30 to 160 bar, preferably 30 to 70 bar, at the end of the draw-in and plastification zone of the twin-screw extruder and, in the mixing zone adjoining the draw-in and plastification zone, is homogenized with the polymer melt to form an expandable polymer melt and cooled to the outlet temperature. In accordance with a further proposal of the invention the method is performed in such a way that the twin-screw extruder is provided with such an amount of polymer that it is operated with a standing column or underfed. By means of this the mixing of the polymer with the nucleating agent and the physical foaming agent can be intensified, and thus an even better result can be achieved.

[0031] Twin-screw extruders with screw diameters of preferably 40 to 130 mm, particularly preferred are 65 to 125 mm, can be employed for the method and the device in accordance with the invention. The draw-in and plastification zone is designed in a length corresponding to approximately 10 to 15 D in order to assure the complete melting of the polymer provided with the nucleating agent. By means of the particularly intensive mixing operations in the twin-screw extruder, the method in accordance with the invention makes it possible to design the special mixing zone, in which the physical foaming agent is introduced into the polymer melt, very short, namely approximately 3 to 6 D, and yet to obtain a particularly even structure and a good surface quality of the foamed foil. With a total length of the twin-screw extruder of approximately 30 to 42 D, a sufficiently long section, embodied as the cooling zone, adjoins this mixing zone in order to cool the expandable polymer melt during the homogenization from the melting temperature of up to 270° C. to a material temperature between 160 and 180° C. prior to its exit from the twin-screw extruder.

[0032] Nucleating agents are advantageously already added to the polymer while it is being fed to the twin-screw extruder in order to make the subsequent foaming easier. Different foaming agents are supplied to the polymer melt separately and immediately following each other at the end of the draw-in and plastification zone of the twin-screw extruder.

[0033] To produce the foam foil made of styrene polymers, the expandable polymer is preferably extruded from a ring-shaped nozzle, and the pipe-shaped tube is cooled on the inside and/or the outside after leaving the nozzle, preferably charged with cooling air. This charging can take place, for example, by blowing. Here, the cooling air should have a temperature preferably in the range between 5 and 50° C., preferably below room temperature. It is also possible to blow cooling air against the tube on the outside and/or the inside by means of appropriate cooling rings attached to the nozzle.

[0034] The inside of the foamed tube exiting from the ring-shaped nozzle can furthermore be charged with compressed air for transverse stretching, so that the tube is expanded and the diameter of the pipe-shaped tube is increased and the pipe-shaped tube is stretched. The longitudinal stretching is caused when drawing the foam foil off, i.e. by an appropriately increased draw-off speed in respect to the exit speed from the nozzle. The tube expanded in this way is subsequently conducted over a cooling and calibration mandrel, which further cools the pipe-shaped tube by contact cooling and at the same time calibrates its diameter. The cooling and calibration mandrel is given the right temperature by means of a flow of water through it, which is tempered to a suitable temperature below 65° C., preferably in the range of 25 to 65° C., by means of a tempering apparatus. The expansion ratio which can be achieved with the method of the invention, i.e. the ratio of the diameter of the outlet opening of the tube from the ring-shaped nozzle to the diameter of the cooling and calibration mandrel is approximately 1:2 to 1:5, depending on the density.

[0035] After the foamed tube has been conducted over the cooling and calibration mandrel and is sufficiently cooled, it is either continuously slit open along one side for forming a flat foil web, or into two foam foil webs on two sides located opposite each other.

[0036] The foam foils formed in this way and laid flat are continuously drawn off by a draw-off device and can either be rolled up or conducted in a line for further processing, for example to a laminating station or a deep-drawing station.

[0037] The foam foil produced in accordance with the method of the invention can directly be laminated on one or both sides with a suitable functional layer, for example a single- or multi-layer compact, i.e. not foamed, foil of a thermoplastic material or another material. Lamination can advantageously follow the production of the foam foil in line.

[0038] Moreover, a particularly advantageous embodiment of the invention is proposed, namely that the expandable polymer melt already injected with gas is coated inside the ring-shaped nozzle prior to exiting it with at least one material, which is compact or expandable and being extruded, for example a thermoplastic material. In this case a further extruder is provided for feeding each thermoplastic material, wherein the outlet nozzle of the twin-screw extruder is designed as a co-extrusion nozzle.

[0039] The invention will be explained in greater detail in what follows by means of an exemplary embodiment and the drawings. Shown are in:

[0040]FIG. 1, a schematic plan view of a device for executing the method,

[0041]FIG. 2, an enlarged partial representation of the partially open-celled foam foil in accordance with the exemplary embodiment.

[0042] As represented in FIG. 1, the device for producing a foam foil made of styrene polymer comprises a twin-screw extruder 1 having a cylinder with identical parallel screws on the inside, which rotate in the same direction, and to whose outlet end a preferably ring-shaped nozzle 12 is attached. The twin-screw extruder 1 is driven by a drive mechanism 2.

[0043] The polymer used for producing the foamed foil, for example polystyrene, is conveyed from a storage facility 13 via a conveyor E2 to a metering device 3, in which it is mixed with the nucleating agent, foreign polymer, surfactant and possibly further additives. This mixture is moved into the twin-screw extruder 1 from the metering device 3 through the draw-in opening 11 in the cylinder.

[0044] A draw-in and plastification zone extends from the draw-in opening 11 of the twin-screw extruder 1 in the working direction A of the twin-screw extruder 1 and has a length L1 of approximately 10 to 15 D, wherein D is the screw diameter. The polymer is heated in this draw-in and plastification zone to temperatures no higher than 270° C. and is melted, and is completely plastified at the end of the draw-in and plastification zone.

[0045] At the end of the draw-in and plastification zone the physical foaming agent, which is fed in via a line E1 by means of a conveyor, not represented, is introduced into the polymer melt through an opening 10 in the cylinder of the twin-screw extruder 1. The polymer melt already injected with gas in this way is subsequently intensely mixed through by the screws of the twin-screw extruder 1 rotating in the same direction in the mixing zone M of the twin-screw extruder 1, which adjoins the opening and has a length L2 of approximately 3 to 6 D, so that an expandable polymer melt with a particularly even and fine distribution of the foaming agent is obtained.

[0046] In the cooling zone of the twin-screw extruder 1 of a length L3 adjoining the mixing zone M, the expandable melt is further homogenized and mixed, wherein it is tempered to a temperature suitable for exiting through the nozzle by means of the appropriate cooling of the cylinder of the twin screw in the cooling zone, i.e. to a temperature of approximately 160 to 180° C. when it exits through the ring-shaped nozzle 12. The total length L of the twin-screw extruder 1 preferably is 30 to 42 D, wherein D is selected to be in the range of 50 to 130.

[0047] The expandable melt exits the ring-shaped nozzle 12 as a pipe-shaped tube S and is cooled on the outside by cooling air 5 of ambient temperature, preferably below 10° C., being blown against it.

[0048] At the same time the tube S is inflated by the compressed air brought in in accordance with the arrow B and blown opposite the exit direction of the tube S into the interior of the latter and is cooled, i.e. its diameter is increased. Thereafter the tube S with its increased diameter is drawn over a cooling and calibration mandrel 6. The cooling and calibration mandrel 6 is provided with a cooling medium by a tempering device, so that the tube S undergoes further interior cooling by its contact with the cooling and calibration mandrel 6. The expansion ratio, i.e. the diameter of the outlet opening of the tube S from the ring-shaped nozzle 12 to the cooling and calibration mandrel 6 is approximately 1:2 to 1:5.

[0049] Following further cooling of the tube S at the exterior surfaces of the cooling and calibration mandrel 6, the tube S has solidified sufficiently so that it can be cut open in the longitudinal direction at two oppositely located areas by means of a cutting device, not represented, for example knives. In the process, two foam foil webs are formed from the tube S, which are laid flat and are drawn off by a draw-off device, not represented, containing several draw-off rollers.

[0050] It is moreover possible by means of the appropriate selection of the draw-off device, not represented, to stretch the solidified, but not yet hardened foam foils in their longitudinal direction in order to prevent the formation of folds and to obtain a smooth surface.

[0051] To produce foamed foils, which are to be coated with a compact or foamed foil of a different thermoplastic material, such as can be required for packaging of foodstuffs or insulations, the ring-shaped nozzle 12 can moreover be designed as a co-extrusion nozzle for coating the polymer melt already injected with gas with at least one other material prior to its exit from the nozzle 12, wherein a further so-called co-extruder is advantageously employed for feeding in any further materials.

EXAMPLE

[0052] A formulation consisting of 100 parts by weight of styrene polymer, 1.4 parts by weight of a nucleating agent of a mixture containing 55 wt.-% of citric acid and bicarbonate and of a decomposition temperature higher than 160° C., and 45 wt.-% of an acrylate-polymer binder, as well as 2.5 parts by weight of an alkyl sulfonate as the surfactant, and 1.9 parts by weight of HDPE granules as the foreign polymer, was fed at a screw rotation rate of 30 rpm into a twin-screw extruder with two identical parallel screws, which rotate in the same direction, each of a diameter (D) of 85 mm, via a metering device at 200 to 250 kg/h. The polymer was completely plastified in the draw-in and plastification zone of a length of 15 D and heated to a temperature between 220 and 270° C. At the end of the draw-in and plastification zone at a temperature of approximately 220° C., 3.5 parts by weight of n-butane as the physical foaming agents were injected at a pressure of 65 bar into the polymer melt, and the polymer melt injected with gas in this way was mixed in a mixing zone of a length of 4 D. Thereafter the polymer melt injected with gas was tempered during continued mixing in the cooling zone of approximately 14 D length until the exit from the nozzle and was brought to a temperature of approximately 167° C. in the cylinder prior to exiting through the nozzle. The expandable molten mass was extruded at a temperature of approximately 167° C. at a pressure of 40 bar through the ring nozzle, wherein the nozzle gap was 1.4 mm. The exiting expanding tube was inflated at a ratio of 1:2.5 with compressed air at room temperature and was subsequently drawn off at a draw-off speed of 11 m/min over a cooling and calibration mandrel cooled by a tempering device, wherein during inflation the outside of the tube was charged with cooling air at room temperature of 15° C. or less. After cooling to a temperature below 150° C. and solidification, the tube was cut on both sides in the longitudinal direction, and the foam foils obtained in this way were laid flat and subsequently rolled up.

[0053] The foam foil obtained has a thickness of 2.5 mm, a bulk density of 0.07 g/cm³ and a partially open-celled structure with a mean diameter of the cells between 500 and 700 mm. Following the opening of the surface skin by perforation, the foam foil is capable of absorbing at least six times its own weight of water.

[0054] An enlarged portion from the interior of a foam foil in accordance with the example with a partially open cell structure is represented in FIG. 2, wherein 100 μm correspond to 1 cm in the scale. 

1. A method for producing an at least partially open- celled foam foil from styrene polymers, characterized in that a mixture containing 0.5 to 3 parts by weight of at least one nucleating agent, 0.5 to 3.5 parts per weight of at least one surfactant, and 1 to 3 parts per weight of a foreign polymer, which cannot be mixed with polystyrene, per 100 parts per weight of styrene polymer, is melted in a twin-screw extruder with screws rotating in the same direction at a temperature of up to 270° C. and 2 to 6 wt.-% of at least one physical foaming agent in relation to the amount of styrene polymer employed is injected into the polymer melt at an injection pressure, which lies above the cylinder pressure of the twin-screw extruder at the injection point and is higher than that for dissolving the foaming agent in the polymer, or sufficiently high for making possible a homogeneously distributed, two-phase system of melt/foaming agent, preferably at a pressure between 30 and 160 bar, and the polymer melt is brought to a material temperature between 160 and 180° C. while being homogeneously mixed and prior to moving out of the twin-screw extruder into the nozzle tool with an annular nozzle, and is extruded under a pressure of up to 55 bar out of the nozzle into the atmosphere in the form of a tube, the tube is cooled on the inside and the outside while expanding and is pulled over a cooling and calibration mandrel, and is cut open after it has solidified and is laid flat as the foamed foil.
 2. The method in accordance with claim 1, characterized in that a foam foil with a proportion of open cells of at least 25% is obtained.
 3. The method in accordance with claim 1 or 2, characterized in that a foam foil of a density of 40 to 100 kg/m³ or greater is obtained.
 4. The method in accordance with one of claims 1 to 3, characterized in that foam foils of a thickness of 0.3 to 10 mm, preferably 1 to 5 mm, are obtained.
 5. The method in accordance with one of claims 1 to 4, characterized in that organic foaming agents from the group of aliphatic hydrocarbons with 1 to 9 carbon atoms, such as methane, ethane, propane, n-butane, isobutane, N-pentane, isopentane and/or neopentane, and/or inorganic foaming agents such as carbon dioxide, nitrogen, are employed as physical foaming agents.
 6. The method in accordance with one of claims 1 to 5, characterized in that calcium carbonate, calcium stereate, talcum, titanium dioxide, silica, barium stereate, diatomaceous earth and/or mixtures of citric acid and sodium bicarbonate are used as nucleating agents.
 7. The method in accordance with one of claims 1 to 6, characterized in that surfactants on the basis of alkane sulfonate are employed for increasing the absorption capability for polar fluids.
 8. The method in accordance with one of claims 1 to 7, characterized in that auxiliary materials, such as inorganic fillers, pigments, antioxidants, UV absorbers, flame-protection agents, soot, anti-static agents, processing aids, lubricants and/or waxes are admixed to the polymer mix.
 9. The method in accordance with one of claims 1 to 8, characterized in that a polyolefin or ethylene-vinyl acetate copolymer is used as the foreign polymer.
 10. The method in accordance with one of claims 1 to 9, characterized in that the tube exiting the nozzle and expanding is cooled on the outside and/or inside by means of cooling air at a temperature at or below room temperature.
 11. The method in accordance with one of claims 1 to 10, characterized in that the tube exiting the nozzle and expanding is cooled on the inside by means of the cooling and calibration mandrel, which has been tempered to a temperature of 25 to 65° C.
 12. The method in accordance with one of claims 1 to 10, characterized in that melting of the styrene polymer, injection of the physical foaming agent into the polymer melt, and cooling of the expandable polymer melt already injected with gas, which is obtained in this way, to the outlet temperature is performed in a twin-screw extruder with a draw-in and plastification zone, mixing zone and cooling zone and a ring-shaped outlet nozzle of a length of at least 25 D, in particular 30 to 42 D (D=screw diameter) with identical parallel screws rotating in the same direction, and the physical foaming agent is introduced into the polymer melt at a pressure of less than 160 bar, in particular between 30 to 160 bar, preferably 30 to 70 bar, at the end of the draw-in and plastification zone of the twin-screw extruder and, in the mixing zone adjoining the draw-in and plastification zone, is homogenized with the polymer melt to form an expandable polymer melt and cooled to the outlet temperature.
 13. The method in accordance with one of claims 1 to 12, characterized in that the twin-screw extruder is supplied with such an amount of polymer that it is operated underfed.
 14. The method in accordance with one of claims 1 to 13, characterized in that, if two or more different foaming agents are employed, they are fed separately and immediately following each other to the polymer melt.
 15. The method in accordance with one of claims 1 to 14, characterized in that the tube which exits the ring-shaped nozzle is charged on the inside and/or the outside with cooling air and with pressure on the inside in order to achieve an expansion ratio of 1:2 to 1:5, and that after expansion the tube is calibrated by means of the cooling and calibration mandrel and is cooled by contact.
 16. The method in accordance with one of claims 1 to 14, characterized in that the tube which exits the nozzle is conducted directly thereafter on the inside and/or the outside to a contact-cooling device via a cooling ring which blows cooling air.
 17. The method in accordance with one of claims 1 to 16, characterized in that the pipe-shaped tube is stretched by a factor of 3 in the longitudinal extension while being drawn off the nozzle.
 18. The method in accordance with one of claims 1 to 17, characterized in that the polymer melt already injected with gas and expandable is coated with at least one compact or expandable material inside the ring-shaped nozzle prior to leaving the nozzle.
 19. The method in accordance with one of claims 1 to 18, characterized in that, after the tube has been cut open, the foam foil, which is laid flat, is laminated with a single- or multi-layer compact or expandable foil on one or both sides.
 20. The method in accordance with one of claims 1 to 19, characterized in that a partially open foam foil is produced from 100 parts by weight of polystyrene, 3 to 4 parts by weight of foaming agent, selected from the group of n-butane, n-pentane, isopentane, or mixtures thereof, 0.6 to 1.5 parts by weight of nucleating agents from sodium bicarbonate and citric acid with a decomposition temperature of greater than 160° C., 1 to 3.5 parts by weight of a surfactant, which reacts well with polar substances such a polystyrene, and 1 to 3 parts by weight of a foreign polymer incapable of mixing with polystyrene, such as a polyolefin or ethylene-vinyl acetate copolymer.
 21. The method in accordance with one of claims 1 to 20, characterized in that the nucleating agent is employed, together with a polymer binding agent on the basis of an acrylate polymer, at a ratio of nucleating agent to polymer binding agent of 55:45 to 65:35.
 22. The method in accordance with one of claims 1 to 21, characterized in that up to 50 wt.-% of a regenerated styrene polymer is used instead of fresh material.
 23. A device for device for producing an at least partially open-celled foam foil on the basis of styrene polymers, nucleating agents, surfactants, physical foaming agents and, if required, further additives, such as foreign polymers and auxiliary materials, comprising a twin-screw extruder with a draw- in and plastification zone, mixing zone and cooling zone, wherein the styrene polymers with the nucleating agent, surfactant, additives are conducted, if required at least partially as a pre- mix, to the twin-screw extruder and are melted to form a polymer melt, and the physical foaming agent is injected under increased pressure into the polymer melt, the polymer melt already injected with gas is cooled to a temperature suitable for foaming and lying below the melt temperature of the styrene polymer, and thereafter exits into the atmosphere from the nozzle of the twin-screw extruder in the form of a tube while expanding, the tube is cooled and cut open in the longitudinal direction and laid flat as the foamed foil, characterized in that as the extruder for melting the polymer, mixing the polymer melt with the physical foaming agent and cooling the expandable polymer melt obtained in this manner, a twin-screw extruder of a length of at least 25 D, in particular 30 to 42 D and having identical parallel screws rotating in the same direction is provided, and the physical foaming agent can be introduced into the melted polymer by the conveying device at a sufficient pressure at the end of the draw-in and plastification zone of an approximate length of 10 to 15 D of the twin-screw extruder and can be mixed with the molten polymer in the mixing zone of a length corresponding to 3 to 6 D of the twin-screw extruder adjoining the draw-in and plastification zone to form an expandable polymer melt.
 24. The device in accordance with claim 23, characterized in that a ring-shaped nozzle with a cooling ring arranged on its outside and/or inside is provided for contact cooling of the tube exiting from the ring-shaped nozzle.
 25. The device in accordance with one of claims 23 or 24, characterized in that a co-extrusion nozzle is provided as the ring-shaped nozzle for coating the polymer melt, which has been injected with gas and is expandable, inside the nozzle with at least one compact or expandable thermoplastic material, wherein respective further extruders are provided to feeding each thermoplastic material into the co-extrusion nozzle. 